1. Technical Field
This disclosure relates to a pigment dispersion suitably used for inkjet printing, an inkjet ink using the pigment dispersion, which enables to solve conventional problems with inkjet head-clogs and allow for obtaining high density images and is excellent in storage stability of ink liquid. The present invention also relates to an image forming process and an image forming apparatus.
2. Description of the Related Art
Inkjet recording method has advantages in that it facilitates full-color printing because the process is easier and simpler than the other recording methods and it allows for obtaining high-resolution images. As an inkjet ink, a dye-based ink is typically used in which various water-soluble dyes are dissolved in water or a mixed liquid containing water and an organic solvent, however, dye-based inks have a disadvantage in that they are poor in light resistance, although they are excellent in color tone definition. In the meanwhile, a pigment-based ink in which a carbon black or various organic pigments are dispersed is now actively studied because of its excellence in light resistance, as compared to dye-based inks. However, pigment inks tend to easily cause nozzle clogs as compared to dye-based inks.
Typically, a pigment-based ink is prepared by first preparing a dispersion in which a pigment and a dispersing agent are preliminarily dispersed in a water-soluble solvent such as water and alcohols, next dispersing the dispersion to a certain degree using a medium-type dispersing device such as sand mill, and then diluting the dispersion to a specific concentration.
Typically, in a pigment water-based ink, a hydrophobic pigment is dispersed, and thus it is usual to use a dispersing agent such as surfactant and water-soluble resin, however, there is a problem that reliability of quality of obtained images with the use of such a pigment water based ink is extremely poor. Then, aiming to improve image quality, a technique is disclosed in which a film-formable resin fine particle is added in an ink liquid. However, it is difficult to finely and stably disperse a plurality of components in a liquid for a long period of time. When a large amount of a dispersing agent such as surfactant is used to stably disperse these fine particles, there are problems with occurrence of air bubbles in an ink tank and/or an ink head, degradation in image quality and the like. To improve dispersibility of these components, there have been the following methods considered: (1) a method of changing the surface of a pigment to a surface containing a hydrophobic group(s), (2) a method of using a resin containing a hydrophobic group(s), and the like. However, employment of such a method causes a problem that when different type components are mixed, the components are unstably dispersed and the storage stability of the ink is degraded, although when one of these components is singularly used, it can be stably dispersed in a water-based ink liquid.
In addition, for the purpose of stabilizing the dispersibility of these components in a water-based ink liquid, there have been the following methods proposed. For example, a method in which a water-dispersible resin having a carboxyl group and a nonionic hydrophobic group in a molecule is dispersed in water (see Japanese Patent Application Laid-Open (JP-A) No. 5-239392); a method in which the polarity of a water-soluble polymer is uniformed to that of a surfactant or a nonionic surfactant is added in a water-based liquid (see Japanese Patent Application Laid-Open (JP-A) No. 8-283633); a method in which in an aqueous recording liquid, the polarity of a color ionic group-containing polyester resin is unified to that of hydrophobic groups of colorants (see Japanese Patent Application Laid-Open (JP-A) No. 2000-63727); and a method in which the polarity of pigments is unified to that of a resin fine particle (see Japanese Patent Application Laid-Open (JP-A) No. 2001-81366).
Further, a water-based inkjet ink composition is proposed, which contains a pigment dispersion containing a pigment particle having a specific particle size distribution, an aldehyde naphthalenesulfonate dispersion and water (see Japanese Patent Application Laid-Open (JP-A) No. 8-333531).
Japanese Patent Application Laid-Open (JP-A) No. 56-147871 proposes a recording liquid which contains a pigment, a polymer dispersing agent and a nonionic surfactant.
U.S. Pat. Nos. 5,085,698 and 5,221,334 respectively propose using an AB block copolymer or a BAB block copolymer as a dispersing agent for pigments to be used.
Further, U.S. Pat. No. 5,172,133 proposes using a specific pigment, a water-soluble resin and a solvent.
In the meanwhile, as a method of dispersing a pigment without using a dispersing agent, the following methods are proposed. Specifically, a method of inducing substituents containing water-soluble group(s) into a carbon black (see U.S. Pat. No. 5,571,311); a method in which a water-soluble monomer etc. is polymerized to the surface of a carbon black (see Japanese Patent Application Laid-Open (JP-A) No. 8-81646); and a method in which a carbon black is subjected to an oxidative treatment (see Japanese Patent Application Laid-Open (JP-A) No. 8-3498).
A method is also proposed which ensures the water resistance and ejection stability of an ink through the use of an ink containing a carbon black that has been subjected to an oxidative treatment, and a ternary copolymer that contains an acrylic resin, a styrene and an a methyl styrene (see Japanese Patent Application Laid-Open (JP-A) No. 9-194775).
Further, an inkjet recording liquid is proposed in which the volume average particle diameter of a dispersion particle is 30 nm to 200 nm (see Japanese Patent Application Laid-Open (JP-A) No. 2000-144028).
Among the above-noted conventional ink liquids mentioned above, color pigment inks actually allow for obtaining high density images, however, black pigment inks have not yet been able to provide enough performance to satisfy users, and further improvements and developments are desired.